The invention relates to a method of manufacturing a monomode mode optical fibre in which: a preform having a central light-conducting core, a light-conducting cladding layer surrounding the core and optionally a substrate layer surrounding the cladding layer is formed by depositing from the gas phase and optionally then collapsing the core and cladding layer; a non-light conducting material is provided around the preform; and the preform is heated and drawn into a monomode optical fibre.
Methods of this type are known from DE-OS 25 23 401 and 28 06 523 and from U.S. Pat. No. 4,283,213. In these references the known methods of reactive chemical vapor deposition (CVD) from the gaseous phase and rod-in-tube are used. In the rod-in-tube method the rod is the preform and the tube is the non-light conducting material.
In the manufacture of monomode optical fibres on a production scale, the OVD method, the VAD method, the MCVD method, and the PCVD method are used as variations of the CVD method (H. Lydtin "Review of Fiber Manufacturing" J. Lightwave Technology, Vol. LT-4, August, 1986, 1016-1038). These methods may be divided into "outer" and "inner" coating methods. A common feature of these methods is to deposit light-conducting core material and light-conducting cladding material from the gaseous phase--after preceding chemical conversion. In the outer coating methods (OVD and VAD), non-light-conducting outer cladding material is deposited directly from the gaseous phase onto the conducting cladding material, whereas in the inner coating methods (MCVD and PCVD), the non-light conducting material is in the form of pre-manufactured substrate tubes and deposition of the light-conducting material occurs on the tube's inner wall.
By the known methods optical fibres of a high bandwidth can be manufactured, the attenuations of which lie near the theoretical limit for the glass/doping systems to be considered. However, to be able to take advantage of the fibre draw rates of more than 1000 meters per minute achievable nowadays, high through-puts in all manufacturing steps, high material yields, very good process control and preforms which produce several 100 kilometers of monomode optical fibres per meter of preform are required.
The preform sizes now in production produce between 30 and 80 km fibre/meter of preform. In the realization of larger preforms the following difficulties are encountered in the CVD method used on a production scale.
In larger preforms produced by the outer coating method, the mechanical stability leaves much to be desired since the density is only a tenth of the theoretical density. The drying, cleaning and sintering devices must be made so as to fit green dimensions up to 2 m in length and approximately 150 mm diameter. Furthermore, a further increase of the deposition rate to approximately 30 g/min. would be required for this method to be able to compete with other methods of manufacturing the non-light-conducting material.
In the inner coating methods, preforms of approximately 300 km of fibre per meter of preform upon deposition of the light-conducting materials require deposition rates of approximately 5 g/min and substrate tubes having inside diameters of approximately 35 to 40 mm. In the deposition of light-conducting materials large preforms require deposition thicknesses of more than approximately 8 mm which present problems for the MCVD method. Thick depositions in large substrate tubes cannot be collapsed by the known methods.